FIG. 1 shows a metropolitan network 100 with modular photonic switching, with a level of lambda conversion, including a set of service aggregation devices 120, a set of edge photonic switch nodes 130, a set of tandem photonic switch nodes 140 and a core 100 comprising a set of core photonic switch nodes 150. An architecture similar to that of FIG. 1 has been described in aforementioned U.S. patent application Ser. No. 09/893,493.
In the metropolitan network 100 shown in FIG. 1, the core photonic switch nodes 150 can provide transport level grooming of sub-lambda-level (and lambda-level) services and can also provide direct service-level switching of lambda-level services. Furthermore, the core photonic switch nodes 150 can include sub-lambda-granular service-level switches 160 such as core routers (or packet switches), core ATM switches and core TDM switches to provide centralized switching of services at less than an entire lambda capacity. The service aggregation devices 120 are adapted to aggregate packet traffic (destined for a core router or packet switch), ATM traffic (destined for a core ATM switch), and TDM traffic (destined for a core TDM switch) into separate optical carriers, in order to simplify the core 110 and prevent core node capacity explosion. This segregation also allows the optical carriers to be fed directly into the appropriate sub-lambda-granular service-level switch 160 by the core nodes. Alternatively, multiple service types may share a common optical carrier, in which case a stage of electronic sub-wavelength switching should be interposed between the core nodes 150 and the sub-lambda-granular service-level switches 160 in order to route different components of the common optical carrier to different ones of the sub-lambda-granular service-level switches 160.
Each of the photonic switch nodes 130, 140, 150, includes a photonic switch and an optical carrier conditioning/validation sub-system (not shown in FIG. 1) for providing wavelength-level switching of optical signals. Thus, the metropolitan network 100 provides switchable express photonic pipes between the service aggregation systems 120 and the core 110. By using this photonic lambda switching architecture, in conjunction with electro-optical sub-lambda switching nodes at the core locations, the complexity of the electronic and electro-optic components of the network 100 is fundamentally minimized, requiring only one stage of electronic multiplexing, one stage of electro-optic conversion, one stage of opto-electronic conversion on the way to the service-level switch 160, one stage of service-level switching within the metro area, and then a return path to the far end metro customer. This leads to a much lower cost and complexity for the electro-optic and electronic parts of the network 100 but introduces the requirement for a photonic infrastructure. As such, the electronic and electro-optic complexity of the network 100 is fundamentally minimized through the use of photonic functionality to provide the appropriate connections.
In addition, the use of photonic paths that completely bypass the electronic and electro-optic components of the core nodes will permit the establishment of all-photonic end-to-end switched connections. Initially, it is expected that such end-to-end photonic paths will be very uncommon, due to the extreme bandwidth requirements to make them economically practical, the prove-in currently being situated at about 3 to 4 gigabits per second (Gb/s) per connection. However, as time progresses and optical integration becomes practicable, the prove-in is expected to drop to around 150 Mb/s, at which point many end-to-end transport pipes can be directly provisioned in a photonic fashion. The provisioning of end-to-end optical transport pipes will require an ability to change the wavelengths of optical carriers in the network, i.e., to move optical carriers from one wavelength slot to another, analogous to moving timeslots in a TDM switched network. This process, often called wavelength (or lambda) conversion, is required at some, but not necessarily all network nodes.
With continued reference to the network 100 in FIG. 1, each core node 150 includes a wavelength-level switch 155 and one or more (electrical) service-level switches 160 such as IP/packet switches, TDM/SONET switches and STS cross-connects. In addition, the core photonic switch nodes 150 may include or otherwise be connected to legacy equipment such as the TDM telephony network, and provide connectivity to long haul (LH) gateways. At least two core photonic switch nodes 150 are typically required in the network 100 and usually a greater number are provided. Exactly two core photonic switch nodes 150 give survivability, while more than two give scalability and offer protection savings.
The core photonic switch node 150 serves not only to connect each incoming wavelength to the appropriate sub-lambda-granular service-level switch 160 for a given wavelength payload, but also to select the correct capacity port on that sub-lambda-granular service-level switch 160 so as to avoid stranding core resources. As such, the wavelength-level switch 155 of the core photonic switch node 150 provides wavelength-level connectivity between buildings and electronic protocol-specific or service-specific boxes. Otherwise, the provisioning granularity would be at the fiber level, precluding the advantageous use of dense wavelength division multiplexing and demultiplexing. By the same token, exploitation of the wavelength-level switch 155 to its full potential requires some level of segregation with respect to the wavelengths traveled by IP and TDM traffic within the network 100. It is also noted that the wavelength-level switch 155 may provide dynamic network load balancing and protection in case of network failures.
Regarding the edge photonic switch nodes 130, these provide the ingress and egress points into the metropolitan network 100. The edge photonic switch nodes 130 are typically located in office buildings, although they may appear elsewhere. The optical signals migrate from sparse DWDM (S-DWDM) into DWDM by an interleaving process and continue their path across the network 100 (see above-mentioned U.S. patent application Ser. No. 09/893,498 and U.S. patent application Ser. No. 09/972,989). The location where the access (S-DWDM) plant meets the inter-office plant is the edge Central Office. The edge photonic switch nodes 130 can be planar in nature, since there is no substantial need for wavelength conversion anywhere but in the core 110. Wavelength conversion is only applied in the case of intra-metro end-to-end wavelength circuits and this can be done in the tandem photonic switch nodes 140 or in the core photonic switch nodes 150. Initially, only rarely will the lack of wavelength conversion capability at the edge node 130 result in a wavelength that could have been locally switched being sent to a wavelength-conversion-equipped node instead, although the prevalence of this occurrence will increase somewhat over time. However, the changed community of interest statistics of the evolved data network, relative to the old telephony network (with its preponderance of local calling), means that the lack of local lambda conversion to complete a local photonic connection and the consequent need for back-haul to a node that does have lambda conversion does not become a problem, since only a relatively small percentage of traffic will be back-hauled when it could have been locally converted.
For its part, a tandem photonic switch node 140 provides a number of functions including a further point of partial fill consolidation before reaching the core 110, establishing end-to-end wavelength paths with wavelength conversion where required and providing a flexibility point for the addition of more core photonic switch nodes 150 or edge photonic switch nodes 130 without having to add dedicated core-edge paths. In addition, the tandem photonic switch nodes 140 may also operate in concert with the edge photonic switch nodes 130 and core photonic switch nodes 150 to provide dynamic traffic load balancing, protection and restoration functions against equipment failure or cable cuts in the core network. A level of wavelength conversion in the tandem photonic switch nodes 140 is beneficial in order to accommodate back-hauled intra-metro wavelength services without routing them back to the core photonic switch nodes 150. All other services/circuits travel to the core photonic switch nodes 150 since this is where the long haul gateways and the sub-wavelength switching functions are located.
As can be appreciated from the above, there is little need for wavelength conversion anywhere but in the tandem photonic switch nodes 140 and core photonic switch nodes 150. It has been estimated that once the metropolitan network 100 is used exclusively for photonic end-to-end connections, only in the case of about 5–10% of wavelengths will the lack of wavelength conversion capability at an edge photonic switch node 130 result in a wavelength that could have been locally switched being sent to a wavelength-conversion-equipped node instead. Stated differently, an edge photonic switch node 130 would ideally be required to provide about 5–10% wavelength conversion under expected future traffic conditions. On the other hand, analyses have shown that the tandem photonic switch nodes 140 and core photonic switch nodes 150 will need to be able to convert as much as 30% and 70% of their incoming wavelengths, respectively, in order to provide satisfactory performance under expected future traffic conditions, once the network 100 has transitioned to the provision of photonic end-to-end paths. In the meantime, the numbers will be somewhat lower, due to the need to terminate most optical carriers in the electro-optic structure of the core photonic switch node 150.
Furthermore, as the photonic switched network shown as 100 in FIG. 1 evolves and thereby grows to accommodate increased traffic, both the number of nodes and the size of those nodes will have to evolve. Thus, the nodes of a practical network have to be sized both initially and in terms of growth, to the actual network traffic levels of provisioned traffic through each individual node, precluding a “one size fits all” approach or approaches which do not scale well, both up and down in size.
These requirements place a fundamental demand on the nodes to be flexible in terms of overall capacity and to permit both various initial node throughputs and various different growth rates. This can only be achieved with a scalable, modular node since for non-scalable nodes size increase requires a “fork-lift” upgrade with the attendant massive disruption to the network at that node site.
Thus, there exists a need in the industry to provide a modular, scalable photonic switch that exhibits desirable blocking performance on both its through paths and its wavelength conversion paths even when heavily loaded and even when a significant percentage of incoming wavelengths need to be converted.
Some conventional photonic switches can be highly modular but exhibit poor blocking performance, as is the case with the switch described in aforementioned U.S. patent application Ser. No. 09/511,065. Such switches are based on a per-wavelength switching structure and hence are highly modular. However, they only provide sufficient wavelength conversion capability to handle the pure edge photonic switch applications and some hybrid edge-tandem photonic switch nodes up to the case where a few percent of incoming carriers must undergo wavelength conversion. However, the longer-term (and in some cases near-term) tandem photonic switch node and core photonic switch node wavelength conversion capability requirements are beyond the reach of the per-wavelength switch, since the latter exhibits a significant blocking probability, especially when heavily loaded.
Other conventional switches can exhibit superior blocking performance but are highly non-modular and non-scalable. Such is the case with the LambdaRouter™ all-optical switch from Lucent Technologies, which is an any-to-any switch based upon a large 3-D MEMS mirror chamber. The any-to-any property exhibits low blocking. However, such switches generally do not provide a wide range of sizes combined with scalability and modularity, generally requiring either a massively over-provisioned initial switch core or a “fork-lift” upgrade once the core runs out of capacity. These factors, combined with the complexity and expense of achieving a functional solution, have prevented any-to-any switches from achieving practicality.
Clearly, there still exists a need in the industry to provide a modular photonic switch with wavelength conversion that exhibits desirable blocking performance even when heavily loaded and even when a significant percentage of incoming carriers need to undergo wavelength conversion.